The present invention relates to a spectrometer apparatus for separating light radiation into a plurality of spectral bands and, more particularly, to a fiber optic polychromator employing optoelectronic detector means for performing multi-element spectral analysis.
Multielement analysis requires a wide range of spectra to be covered simultaneously as emissions from multielement samples commonly span the ultraviolet and the infrared spectral regions. Various types of spectrometers and polychromators are useful in this work, particularly those having multientrance slit arrangements. In one proposed arrangement light radiation is guided to each of a series of horizontally spaced entrance slits of a polychromator by optical fibers. These fibers are commonly made of glass or quartz and one end of each optical fiber can be selectively positioned near a light source while the other end can be aligned with any one of the entrance slits to guide radiation from a selected part of the light source to the polychromator for separation into a spectral bands. (defined here as the spectral region simultaneously covered by the detector). In this proposed use of optical fibers with a horizontal arrangement of spaced entrance slits, an optoelectronic multichannel vidicon detector may be employed. One such vidicon may be, for example, an "OMA" silicon intensified target (SIT) detector described in catalogue No. T388-15M-5/78-CP published by Princeton Applied Research Corp., Princeton, N.J. Such a vidicon detector generally includes a two-dimensional photosensor target and an electron gun for effecting a target readout scan.
In using optical fibers with multiple entrance slits, each entrance slit may operate to focus a particular spectral region or band (e.g., 40 nm.) on the vidicon target. Numerous advantages are realized by use of such a multi-slit arrangement. The use of optical fibers lends flexibility to the collection of radiation. The source end of an optical fiber can be located at any position adjacent a light source to gather the desired radiation while the other end of the fiber can be inserted into any of the multiple entrance slits to display the desired spectral band.
A problem inherent in this form of spectrometer, however, is that it provides only a one-dimensional display of spectra whereupon overlapping of spectra occurs when more than one optical fiber is inserted into the entrance slit arrangement. Analysis of overlapping spectra is both difficult and generally inaccurate. Another disadvantage is that such a one-dimensional display of spectra does not result in an efficient use of the two-dimensional vidicon target, which is a potentially powerful tool for multielement trace analysis.
One known form of spectrometer system known as the "echelle-crossed dispersion" system is capable of displaying spectral bands in a two-dimensional configuration. This system, however, employs complex optics and does not display the spectra in parallel bands. Furthermore, the bands are non-uniform in size which makes detection difficult and inefficient since large portions of the target are not utilized. Also separation between parallel bands is not uniform. Resolution also varies across the spectra in this type of system and furthermore in order to utilize optoelectronic detector apparatus the non-parallel display of bands calls for a complex computer-controlled detector system and constitutes an inherently inefficient use of the two-dimensional vidicon target. Furthermore, this system is notorious for its high stray light levels, a direct result of the optical design and configuration.